Glutamate release evoked by glutamate receptor agonists in cultured chick retina cells: modulation by arachidonic...

We studied the effect of ionotropic glutamate receptor agonists on the release of endogenous glutamate or of [3H]D-aspartate from reaggregate cultures (retinospheroids) or from monolayer cultures of chick retinal cells, respectively. Kainate increased the fluorescence ratio of the Na+ indicator SBFI and stimulated a dose-dependent release of glutamate in low (0.1 mM) Ca2+ medium, as measured using a fluorometric assay. Under the same experimental conditions, the release evoked by N-methyl-D-aspartate (NMDA; 400 microM) was about half of that evoked by the same kainate concentration; alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA; 400 microM) did not trigger a significant response. In the presence of 1 mM CaCl2, all of the agonists increased the [Ca2+]i, as determined with the fluorescence dye Indo-1, but the glutamate release evoked by NMDA and kainate was significantly lower than that measured in 0.1 mM CaCl2 medium. Inhibition by Ca2+ of the kainate-stimulated release of glutamate was partially reversed by the phospholipase A2 inhibitor oleiloxyethyl phosphorylcholine (OPC), suggesting that the effect was mediated by the release of arachidonic acid, which inhibits the glutamate carrier. Accordingly, kainate, NMDA, and AMPA stimulated a Ca(2+)-dependent release of [3H]arachidonic acid, and the direct addition of the exogenous fatty acid to the medium decreased the release of glutamate evoked by kainate in low (0.1 mM) CaCl2 medium. In monolayer cultures, we showed that NMDA, kainate, and AMPA also stimulated the release of [3H]D-aspartate, but in this case release in the presence of 1 mM CaCl2 was significantly higher than that evoked in media with no added Ca2+. The ranking order of efficacy for stimulation of Ca(2+)-dependent release of [3H]D-aspartate was NMDA > > kainate > AMPA.